[twawaaay]

There is only one reason the sea can rise currently due to weather and that is glaciers getting melted.

Another reason (but not due to weather) is that when land somewhere goes up, it will displace water everywhere else. And so, for example, if the land is still recovering from the ice age we should see ocean levels going up everywhere except for the pieces of land that are recovering from the weight of the glacier that is no longer there.

Mind that Arctic is not causing sea level rise. Any ice that is floating on water will not cause any water level change when it melts. (I know this is somewhat unintuitive but it comes directly from Archimedes principle)

So we are talking basically Antarctic ice and Greenland because these are by far the largest bodies of frozen water that are supported by land rather than floating on the ocean.

I think it should be pretty easy to observe how much of that water melted or slipped into the sea.

I also think that currently, coastal erosion is mostly caused by changing weather patterns. Basically this comes down to wind blowing in different directions, speed and variety and these changing patterns mean coasts are eroding in different places than before.

[locopati]

Also, warming oceans causes expansion which causes sea level rise.

https://www.jpl.nasa.gov/edu/learn/project/how-warming-water....

[twawaaay]

Not true.

First of all, most of the temperature rise only happening close to the surface with average surface temperature rise being only about 1.5F or 1C since 1901.

This article says deep ocean water is projected to warm up by only 0.2C during next 50 years: https://www.openaccessgovernment.org/deep-ocean-warming-subt...

Furthermore, at around 4C which is what deep ocean water is close to (everything below 200m is essentially 4C), thermal expansion is almost nil. For colder water thermal expansion is actually negative.

4C is when water is at its densest. It is not an accident that all oceans are 4C, because 4C water sinks to the bottom and anything colder or hotter than 4C floats up. This remarkable property of water is what causes even shallow water to be fantastically stable in temperature -- a lake that has more than couple tens of meters in depth is likely to be 4C at the bottom throughout the year whether it is frosty winter or hot summer above it, unless some kind of powerful event is able to mix the water in the lake.

Now, the small temperature differences will definitely have outsize effects on water circulation, ocean currents, life and weather. But I doubt they will cause meaningful sea rise unless somebody can calculate otherwise?

[Retric]

That’s mostly accurate, but the nuances are significant and lead to different conclusions. For example the hypolimnion may be much warmer than 4C in lakes in warmer areas. More importantly tropical ocean water is above 4C down to roughly 2km and not only is that depth expected to increase, but also the depth of warm water as you go north. https://en.wikipedia.org/wiki/Thermocline

The important thing to remember is even a 1 part in 1,000 decrease in density * 2km of depth = 2m of expansion. Ballpark estimates aren’t enough you really need fairly detailed simulations to get any significant accuracy. Actually doing such simulations shows meaningful sea level rise from thermal expansion at ~0.07 inches per year or roughly half the current rate of increase. This might not sound like much, but consider that volume of sand you need to replace to maintain beaches etc etc.

[kergonath]

> That’s mostly accurate

No, that is mostly inaccurate. Thermal expansion is small, but there is an awful lot of water. As you point out yourself, thermal expansion contributes about half the sea level rise. Oceans absorb energy just like the atmosphere does and this effect has been known for quite a while (e.g. https://www.nature.com/articles/330127a0 ).

[mcguire]

The average ocean surface temperature is about 20°C and the thermal expansion coefficient is 0.000207/°C (https://www.engineeringtoolbox.com/water-density-specific-we...). If I have my google-fu and math right, that's about 1cm/° for a 50m deep water column.

Thermal expansion of surface water is not negligible.

[Tuna-Fish]

> But I doubt they will cause meaningful sea rise unless somebody can calculate otherwise?

Plenty of people have, in fact, calculated otherwise.

Roughly half of the current, ongoing sea level rise is from expansion of water. The effect is small, but there is a lot of water that is expanding.

[jdontillman]

Fascinating; thanks.

Question: How does the salinity and pressure of deep sea water affect that 4C point?

[atlantic]

Not necessarily. The density of water peaks around 4ºC. So as oceans warm between 0º and 4º, they actually contract in volume.

[twawaaay]

Except water in oceans is at 4C (at least almost everything deeper than ca 200m). And in the vicinity of 4C the thermal expansion is very negligible. This graph should explain why: https://images.app.goo.gl/FXzvTkPvE9dYxoUA7

[atlantic]

Which supports the broader point that thermal expansion of water does not contribute to rising sea levels.

[withinboredom]

Umm. I’m pretty sure if water that exists above the water line is melted into the water line, the overall water line will rise. You can directly observe this in a glass of ice that starts with no liquid water will melt into a glass of liquid water.

[nrclark]

I could be wrong about this (it's been awhile since I took chemistry), but I think the ice has to be floating for the Archimedes principle to apply.

You can fill a glass of ice-water right up to the brim, and it won't spill over as the ice melts. But only if the ice is floating in the water. It's because the ice's mass pushes down on the liquid water, displacing a fixed amount relative to the weight of the ice.

[withinboredom]

Ah. I missed the “floating” part. That makes sense.

[brilee]

This is untrue.

Ice is about 91% the density of water. If you have water at 0 height, and you put in water equivalent to +1 unit of height. If it's in liquid form, obviously height goes up by +1. If it's in ice form, there is +1.09 height worth of ice (because it expands when it freezes), but it only displaces water up to +1 unit of height in order to support its weight through buoyancy. The overall change in height is +1 unit regardless of whether it's liquid or ice

[trompetenaccoun]

No, it's not just due to weather (you probably mean climate anyway). For example, ice melting in one place, say Antarctica, will affect sea levels elsewhere on the planet because of weaker gravitational forces where the ice used to be. Geoscience is complex and measuring changes on such a global scale is not "pretty easy", even if it's only sea level changes. It's nothing like a bathtub or elementary school physics.

Not to rant but this is one of those threads again. The majority of comments contain misinformation.